(a) Industrial Field of the Invention
The present invention relates to a ferroelectric liquid crystal composition, a liquid crystal optical device produced by using the ferroelectric liquid crystal composition, and a method of producing the liquid crystal optical device. Particularly, the present invention relates to a ferroelectric liquid crystal composition which may be suitably used as the liquid crystal material for liquid crystal display devices, liquid crystal memory devices, liquid crystal speaker devices, etc., to a liquid crystal optical device produced by using the ferroelectric liquid crystal composition, and to a method of producing the liquid crystal optical device by which the liquid crystal optical device can be produced in a better yield through a simple process.
(b) Description of the Related Art
In recent years, liquid crystal optical devices in which a highly oriented ferroelectric liquid crystal is used as a liquid crystal material and supported between two electroded substrates have been attracting interest because of their excellent qualities, for example, high-speed response to external factors, such as electric field, and high contrast ratio, and accordingly, their use for liquid crystal display devices, liquid crystal memory devices, etc. has extensively increased.
However, in case of producing such liquid crystal optical devices by using a liquid crystal material comprising only ferroelectric liquid crystals, there have been raised problems that cell thickness cannot be increased, resulting in inviting occasional continuity defect, color shading due to birefringence interference, a tendency of bistability to become uneven, and the consequent difficulty in production of devices having large area.
In order to solve the problems, in Japanese Patent Application Kokai Koho No. 62-48789, there disclosed a method of controlling scattering of light by using a liquid crystal device which encloses a liquid crystal material having smectic phase and a medium encapsulating the liquid crystal material. However, the liquid crystal device is slow in responding to electric field (from several tens to several hundreds ms) because in the device, response to electric field is not conducted in smectic phase but in nematic phase, and smectic phase merely serves to maintain the state in nematic phase after the state is transformed from nematic phase by varying temperature. Further, the device has another problem that the contrast is low because light is switched between on-state and off-state by switching the mode of light between scattering state and non-scattering state.
In Japanese Patent Application Kohyo Koho No. 61-502128, there is disclosed a liquid crystal optical device which encloses a liquid crystal material comprising a liquid crystal matter dispersed in a transparent epoxy resin in a state encapsulated by the epoxy resin. The liquid crystal optical device also has the problems that it is slow in responding to electric field because the response to electric field is conducted in nematic phase and that the contrast is low because the switching of light between on-state and off-state is performed by switching the mode of light between scattering state and non-scattering state.
In Japanese Patent Application Kokai Koho No. 63-137211, there is disclosed a disperse system of a liquid crystal dispersed in resin matrix, and the disperse system is used as a blind by switching the light between scattering state and non-scattering state by applying electric field. However, the response time of the disperse system Is also large because nematic liquid crystals are used as the liquid crystal matters, and enough contrast cannot be attained because the light switching system between on-state and off-state is perfomed by switching the light between scattering state and non-scattering state. The disperse system, therefore, is not enough to be used for display devices.
Further, in order to make ferroelectric liquid crystals fully exhibit its excellent qualities, it is necessary to highly orient the liquid crystal material, i.e. the ferroelectric liquid crystal, and therefore, there have been proposed various methods of controlling orientation.
For example, in case where a low molecular weight ferroelectric liquid crystal is used as a ferroelectric liquid crystal, the orientation has been controlled by using a rubbing technique, a shearing technique, a temperature gradient technique, a SiO oblique evaporation technique or the like.
However, production of liquid crystal optical devices using ferroelectric liquid crystals has not been put into practice although such devices are superior in high speed response property and memory capacity to the liquid crystal optical devices which use nematic liquid crystals and have been the mainstream of conventional liquid crystal optical devices. There are some reasons as described below.
(1) It is difficult to enlarge the area of a device since the cell-thickness of a device should be adjusted to not more than several .mu.m.
(2) In the method of controlling orientation by previously coating a substrate with a polymer (polyimide, etc.) and subjecting the polymer layer to rubbing treatment or the like, there are deficiencies such as the complicated pre-operation and pre-adjustment of the substrates, and further, the usual use of glass substrates requires the producing apparatus including complicated conveyer means to be maintained extremely clean and also makes the realization of continuous production difficult.
(3) There are few liquid crystals which exhibit ferroelectric liquid crystal phase (chiral smectic C phase) which is thermodynamically stable at temperatures neighboring room temperature.
For example, in Japanese Patent Application Kokai Koho No. 63-25622, there is disclosed a method for lessening the change of the structure of liquid crystal caused by temperature change, by mixing a ferroelectric low molecular weight liquid crystal with a thermoplastic resin to improve the formability of the liquid crystal material into film and to make it exhibit ferroelectric phase at temperatures neighboring room temperature. However, the method requires specified combination of resins and liquid crystals to attain the thermodynamic stability of ferroelectric phase, and the orienting treatment in the method should be conducted by using rubbing technique or the like which make the process complicated and make enlargement of area difficult. Further, there is a problem that the conventional rubbing technique or the like cannot produce sufficient orientation state. Furthermore, the method is poor in productivity because production of the above-mentioned composite film should be conducted by dry-and-wet film-forming technique, water-surface-spreading technique or the like and therefore, several sheets of the film should be laminated for producing electrically optical devices.
In Japanese Patent Application Kokai Koho No. 63-109418, there is disclosed a composite liquid crystal material consisting of non-liquid-crystalline polymeric material and a ferroelectric low molecular weight liquid crystal. However, because the formability of the composite liquid crystal material into film relies only upon the polymeric material, there are problems that it is difficult to produce thin film of less than several .mu.m in thickness and that the mechanical strength of the produced device is insufficient. There is further problems that the stretching technique, which is employed in the method for controlling the orientation state of liquid crystal, is not enough to highly orient low molecular weight liquid crystals and makes full exhibition of histability difficult because low molecular weight liquid crystals orient along the polymeric material being oriented by stretching.
In Japanese Patent Application Kokai Koho No. 63-109149, there is disclosed another method for obtaining a composite film containing oriented liquid crystal. In the method, a liquid crystal is mixed with a polymeric material to produce a composite film, and the composite film is oriented by uniaxially stretching it and subsequently, heating it to a temperature at which the liquid crystal exhibits isotropic phase and then cooling the heated composite film, whereby omitting some steps including the step of dispersing spacing materials at the time of producing ferroelectric liquid crystal panels, the step of coating orientation layer, the step of rubbing treatment, and the step of injection of liquid crystal. However, since the orienting mechanism employed in the method is similar to rubbing treatment in which the liquid crystal molecules are oriented along the polymer oriented by uniaxial stretching, the method has problems that the productivity is poor because of the essential slow-cooling from isotropic temperature, that the liquid crystal material cannot be highly oriented resulting in the low contrast of the obtained device, that the stretched composite film cannot be supported between two substrates without difficulty because, for example, air bubbles are apt to be caught simultaneously, that the optimum combination of ferroelectric liquid crystals and polymeric materials should be selected because the degree of orientation varies depending on the kinds of ferroelectric liquid crystals and polymeric materials.